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Seal face materials have different properties are chosen based on the application. A tutorial by Brian Kalfrin, Jack Bagain of John Crane at the 2017 Turbomachinery & Pump Symposia threw some light on this topic. This article contains excerpts from their tutorial, "Midstream Pipeline Applications – Design Aspects and Considerations for Mechanical Seals".
Carbon-Graphite – To enhance the properties of carbon-graphite grades, they are typically impregnated with various substances in order to achieve the required chemical and physical properties. These adders are typically resins, ceramics, and metals. Metal impregnated carbons offer the highest strength and antimony is commonly used; Antimony impregnated carbon is the default carbon identified in API 682 when considering use in a light hydrocarbon service. Antimony carbon is commonly used in various services in midstream pipelines. Specialized Nickel impregnated grades have also been used widely with good success, although lead times and costs of those materials make them less desirable options.
Although antimony filled carbon-graphite primary ring can produce the best performance and lowest leakage, the relatively soft nature of the material in abrasive or viscous services can present limitations. In practice, two hard faces are usually necessary in order to prevent mechanical and/or abrasive face damage in crude oil service. Additionally, in pressures beyond 1200 PSIG (82 BARG), evaluation of the carbon material is required to assure sufficient margin of safety against mechanical failure of the material and also deficiencies in the seal performance from excessive distortion due to pressure. The majority of the carbon seal faces utilized in midstream pipeline applications employ engineered ring geometry and configurations to mitigate pressure induced distortion and optimize face contact pressure. The authors of this tutorial have extensively applied engineered carbon face geometry in these pressure ranges.
Sintered / Reaction-Bonded Silicon Carbide - Silicon carbide is an advanced ceramic material. The earliest type of silicon carbide available for use in mechanical seals was reaction bonded and developments have made a number of variations available. Silicon carbide is extremely hard, being highly wear resistant and with good mechanical properties. It has high temperature strength and thermal shock resistance, maintaining its high mechanical strength at temperatures as high as 2550°F (1400°C). Above 2570°F (1410°C) the free silicon melts and strength decays. These advanced ceramics are routinely used in midstream applications as typically the mating face pair with a carbon ring in light hydrocarbon or finished products due to the exceptional PV characteristics of the material pairs. The ceramic materials are typically run against one another (dissimilar grades) in high viscosity applications such as crude oil. Robust drive designs for seal rings of this material are recommended to avoid potential hang-up when contacting the comparatively softer metal components of the seal retainer.
Tungsten Carbide - Cemented tungsten carbides are derived from a high percentage of tungsten carbide particles bonded together by a ductile metal. The common binders used for seal rings are nickel and cobalt. The resultant properties are dependent upon the tungsten matrix and percentage of binder (typically 6 to 12% by weight per volume). Tungsten carbide is an extremely tough material with good wear resistance, with Nickel bound being the most common material used in midstream pipeline applications. Seal rings in this material offer improved protection against mechanical or thermal shock, but will be limited in PV characteristics and are more susceptible to heat checking damage when compared against advanced ceramics.
Silicon Carbide / Graphite Composites - These are sintered silicon carbide composite containing free graphite. The free graphite reduces friction, improving dry run survivability and better thermal shock resistance than conventional sintered materials. Grades are also available with a network of non-interconnecting pores, which entrap fluid to support hydrodynamic lubrication. These materials offer exceptional PV characteristics when paired with a corresponding advanced ceramic material. Seal rings manufactured from graphite / silicon carbide composite materials are typically used in crude oil or finished products and light hydrocarbon service especially when operating pressures may exceed the limits of conventional metal-filled carbon grades.